She said that she is "deeply concerned" about the impact on democracy of the misuse of social media users' personal information, adding that legal systems had failed to keep up with the rapid development of the internet.

Ms Denham said that her office's investigation, triggered by allegations of misuse of Facebook users' personal data, was thought to be the largest undertaken by any data protection authority in the world.

MEPs on the Committee on Civil Liberties, Justice and Home Affairs heard that the Information Commissioner's Office (ICO) investigation covered potential criminal and civil breaches involving more than 30 organisations and dozens of individuals, including social media companies, data brokers, analytical firms, academic institutions and political parties and campaign groups.

More than 40 investigators are working full-time on the ICO probe, backed up by around 20 external legal and forensic digital recovery experts, and have already seized hundreds of terabytes of information on servers and computers.

Mr Wylie told MEPs that senior members of the Leave campaign were now working within Theresa May's administration and wanted to "stall any public inquiry until they have secured Brexit".

He said: "I don't believe Brexit would have happened were it not for the data targeting technology and network of actors set up by Cambridge Analytica.

"I don't believe the Brexit result was won fairly or legitimately."

Mr Wylie added: "If this happened in Nigeria or Zimbabwe, the EU would demand a re-run of the vote. Perhaps we should hold the UK to the same standard."

As well as pursuing specific allegations - many relating to the use of data in the UK's 2016 EU referendum - the ICO is aiming to produce a report by the end of this month on the regulatory and legislative reforms needed to respond to the challenges of targeted online advertising.